When considering traditional refractory materials for primary metallurgical processing of most metals, the oxides of aluminum, Al2O3, silicon, SiO2, and zirconium, ZrO2 are exclusively used. They are embodied in either transient melt containment members, such as pour tubes, or as containment vessels for casting.
Highly reactive metals (e.g., titanium and zirconium) are extremely difficult to handle in their molten state due to their extreme corrosive nature. Many times these materials are processed in water-cooled copper, where a thin layer of molten material solidifies on the water-cooled copper and forming a “skull”. This skull therein provides a boundary layer between the molten material and the water-cooled copper. Much power is required to maintain a melt as significant power/heat is extracted from the water-cooled copper through the skull. This method of processing has been used for both transient melt containment members such as pour tubes, or as casting containment vessels. However, due to the nature of the water-cooled copper, limited geometrical variations are possible.
One other such material that has been tested as a substitute for water-cooled copper casting is stoichiometric rare earth (RE) oxides (i.e., RExOy where RE could be Y, La, or the like). RE oxides are more thermodynamically stable compared to the aforementioned traditional refectory materials (i.e., Al2O3, SiO2, and ZrO2), and thus have been considered as a mold material for the casting of titanium and titanium alloys (see N. M. Griesensuer, S. R. Lyon, C. A., Alexander, “Vacuum induction melting of titanium,” J. Vac. Sci. and Tech., vol. 9, pp. 1351-1355, 1972). This study determined that Y2O3 showed the least amount of reaction with stagnant molten titanium when compared to other RE oxides. As a result of this study, Y2O3 is and has been the main stay of protection when reactive metals are being cast without the use of a water-cooled copper mold. The method of using Y2O3 as a casting mold material is predominantly embodied as a coating on lower cost ceramics such as Al2O3, SiO2, ZrO2 and mixtures thereof. However, Y2O3 does have limitations with respect to its protection for extended times or increased melt superheat (i.e., temperature above the melting point). For example, as indicated in the aforementioned study (Griesensuer et al. 1972), increased time or superheat will cause dissolution of the Y2O3 into the molten titanium causing contamination beyond chemical specification.
For transient melt containment of reactive metals, water-cooled copper nozzles have been utilized for free-fall gas atomization, however, extensive induction and/or plasma power supplies are required to prevent freeze-out and little to no superheat is generated within the melt. One refractory material that has been utilized with moderate success as a transient containment member is graphite. While significant melt superheats are physically possible, significant carbon contamination results and renders the material useless in an industrial perspective. U.S. Pat. No. 6,358,466 describes the only known embodiment of a RE oxide as a transient melt containment member wherein a composite pour tube is fabricated by multiple plasma spray layers.